1. Field of the Invention
The present invention relates to a display device, particularly to a flat display device using minute field emission cathodes.
The minute field emission cathode is an electron source of a higher electron emission efficiency and a higher luminosiy than a thermionic cathode, which is conventionally used, and is a promising electron source for a flat display, an image pickup tube and the like. Particularly, a display device employing the minute field emission cathodes is of a self-luminous type, and can exhibit a higher luminance and a higher resolution. Further, the display device has various excellent characteristics such as wide view angle, quick response and low power consumption.
2. Description of Related Art
Some display devices employing cathodes comprises a vacuum-sealed container in which electrons extracted from field emission cathodes are accelerated by applying an electric field and irradiated onto a fluorescent substance to emit light. As such cathodes, used are conical or planar minute field emission cathodes and surface conduction type cathodes.
FIG. 13 shows an exemplary construction of a display device employing conventional minute field emission cathodes.
The minute field emission cathode includes conical emitter tips 101, gate electrode lines (gate power supply lines) 103 for extracting electrons, emitter electrode lines (emitter power supply lines) 102 for applying a negative voltage to the emitter tips, and an insulation film 104 for isolating the gate electrode lines from the emitter electrode lines. The minute field emission cathode is disposed on a glass substrate 105 as shown in FIG. 14. The entire cathode structure including the glass substrate 105 is herein called a cathode plate 109.
The emitter tip 101 is about one micronmeter in length and is formed by integration on a flat substrate employing a micromachining technique.
The emitter electrode lines 102 and the gate electrode lines 103 are arranged so as to intersect each other. In each intersection, several hundred to several thousand emitter tips are formed on the emitter electrode line. These several hundred to several thousand emitter tips define one display element, e.g., pixel, 106.
When a voltage is applied across the emitter tip 101 and the gate electrode line 103 in vacuum, electrons are extracted from the emitter tip 101 by field emission.
Since the field emission characteristic is non-linear, both the emitter electrode lines and the gate electrode lines can be driven by simple matrix addressing. Electrons extracted from emitter tips in a selected pixel impinge on a transparent anode plate 107 supported in an opposed relation to the minute field emission cathode.
The anode plate 107 has fluorescent layers 108 formed in a stripe pattern on its surface. When electrons impinge on these fluorescent layers 108, the fluorescent layers 108 are excited to emit light. A user observes this light emission through the anode plate 107 or the cathode plate 109.
In order to accelerate electrons, an acceleration voltage of several hundred volts must be applied to the anode plate 107 with respect to the cathode plate. Here, the anode plate and the cathode plate should be spaced about several hundred micrometers for ensuring good insulation therebetween.
Generally, the luminance of a display screen is proportional to the luminous efficiency of a fluorescent layer.
The higher acceleration voltage is applied to the anode, the higher the luminous efficiency becomes, because electrons can penetrate deeper into particles of the fluorescent layer. Therefore, it is preferable for increasing the luminous efficiency to reduce the space between the anode plate and the cathode plate and raise the acceleration voltage applied to the anode.
FIG. 14 shows a sectional view of a display device of a reflection type wherein a cathode plate is placed on a observer's side and light emitted by a fluorescent layer is observed by a user in a reflective form. An acceleration voltage 112 is applied across gate electrode lines 103 and electrically conductive films 110 on an anode plate 107. Electrons emitted by emitter tips 101 is irradiated onto fluorescent layers deposited on the surface of the conductive films 110. Here, the acceleration voltage 112 is about 400V.
Referring to FIG. 14, a plurality of spacers 111 are disposed at proper intervals between the anode plate 107 and the cathode plate 109 to ensure a certain spacing between the anode plate 107 and the cathode plate 109. The inside of a panel defined by the anode plate 107 and the cathode plate 109 is vacuumed. The spacers 111 are formed of a glass material of 200 .mu.m high, and adhered to the anode plate 107 and the cathode plate 109 to support both the anode and cathode plates against atmospheric pressure.
In this construction, if the acceleration voltage 112 is raised higher than 400V, a sudden electric discharge is caused to occure on the surface of the spacers 111 by ions or secondary electrons generated in the panel. This electric discharge often induces destruction of emitter tips 101.
If the spacers 111 are heightened to allow a larger spacing between the anode plate 107 and the cathode plate 109, it increases a creeping distance between the electrode lines 103 of the cathode plate 109 and the conductive films 110 on the anode plate 107 through the surface of the spacers. The increased creeping distance means an increased insulation voltage between the plates, and as a result, the above-mentioned electric discharge and destruction of the emitter tips can be prevented.
However, electrons emitted by the emitter tips 101 travel in a radially spreading manner. Accordingly, the larger the spacing between the plates is, the more a beam of electrons spread, and as a result, resolution deteriorates. Especially in a color display device, electrons expected to be incident on one fluorescent layer may impinge on an adjacent fluorescent layer of a different color, which results in the blurring of colors.
Since the emitter tips 101 cannot be formed in areas where the spacers 111 is disposed, light is not emitted from these areas. Accordingly, in order to obtain a resolution of about 300 .mu.m, for example, the width of the spacers 111 is preferably about 40 .mu.m or less.
In the case where the spacing between the anode plate and the cathode plate is about 200 .mu.m, the insulation voltage is generally about 500V. Accordingly, a spacing of 2 mm is required between the anode and cathode plates in order to obtain a insulation voltage of 5 kv. In this case, an aspect ratio of the spacers 111 is 50:1 (=2 mm:40 .mu.m). It is difficult to form a spacer which exhibits such a high aspect ratio and sufficient strength at the same time. Even if a spacer of this configuration is formed, for example, by cutting a glass fiber, the spacer readily falls down or inclines since the spacer is adhered to the plates only by an extremely small area.
A high aspect ratio can be more easily obtained by forming a spacer in a shape like an elongated wall. In a display as shown in FIG. 14, however, since gas within the panel is discharged in a direction parallel to the plates, a gas discharge conductance declines if wall-like spacers are formed. The decline in the gas discharge conductance results in a fall in a vacuum degree around the emitter tips because the fluorescent layers on which electrons impinge release gas or the like. That brings about defects in display by a decline in the luminance and by the destruction of emitter tips.